1. Field of the Invention
The present invention relates to a supply circuit for an X-ray emitter usable in radiology. In radiology, it is necessary to supply an X-ray source or emitter for a predetermined time. This time corresponds to the exposure time of a radiosensitive plate. In cineradiology, it is also necessary to modify the activity of the X-ray emitter as a function of the timing of recording, so that the radiation dose received by a patient is reduced during such an examination.
2. Description of the Prior Art
Conventionally, the supply circuit for the emitter comprises a transformer, whose primary circuit is connected to a three-phase alternating current network and whose secondary circuit is connected to the emitter. The primary circuit connected in star-like manner has, in place of a nodal connection, a switch for connecting together the three supply conductors to the nodal point. The time during which this switch is closed conditions the putting into operation of the primary and consequently that of the transformer and consequently fixes the activity period of the X-ray emitter. The switch is conventionally provided with a set of thyristors, which are fired during the desired energization. In order to obtain an opening of the switch, the turning off of the thyristors is brought about. For this purpose there is a quantity of electrical energy contained in a previously charged capacitor and which is reverse discharged into the thyristors. When the value of this reverse current exceeds the value of the charging current of said thyristors, the latter are deenergized and are blocked.
Before starting a further firing - turning off cycle of the set of thyristors in question, it is necessary to recharge the blocking capacitor. The duration of this supplementary sequence prevents the repetition of the operating cycle at a high speed, e.g. 50 cycles per second, as used in cineradiography. Moreover, the recharging of the capacitor makes it necessary to provide a supplementary power supply. Finally, during the discharge of the blocking capacitor, the reverse current established in the set of thyristors has a sinusoidal configuration, i.e. it increases up to a maximum and then decreases. In order to block the thyristors, an adequate reverse current must be established there for a time exceeding their covering or overlap time, which leads to the choice of high value capacitors. Thus, the useful duration of the adequate reverse current increases in proportion to the maximum intensity of said reverse current. The latter is dependent on the capacitance of the capacitor. This means that at the strong time of the blocking operation, the thyristors are traversed by a needlessly high current, because it is mainly their overlap or covering time which controls their blocking.